The present invention relates to a fluid rotating apparatus such as a vacuum pump, a compressor, or the like.
FIG. 13 shows an example of a conventional sliding vane vacuum pump provided with only one rotor. In the vacuum pump with one rotor, when the rotor 101 rotates, two blades 102 inserted in the rotor 101 in the diametrical direction of the rotor 101 are driven and rotated inside a cylindrical fixed wall 103 (stator). At this time, the leading ends of the blades 102 are kept in contact with the fixed wall since the blades 102 are always urged in the radical direction of the rotor 101 by the action of a spring 104. Subsequent to the rotation, the capacity of each of the spaces 105 partitioned by the blades 102 in the fixed wall is changed, and a gas entering from a suction port 106 formed at the fixed wall is eventually sucked and compressed and flows out through a discharge port 107 having a discharge valve In the vacuum pump of this type, in order to prevent internal leakage, it is necessary to seal the side surface and the leading ends of the blades 102, the side surface of the fixed wall 103, and the side surface of the rotor 101 with oil membranes, respectively. However, when this kind of vacuum pump is used in the manufacturing process of semiconductors, e.g., CVD or dry etching, etc. using a highly corrosive reactive gas such as chlorine gas, the gas reacts with the sealing oil to thereby generate a reaction product in the pump. In this situation, it becomes necessary to perform maintenance work frequently so as to remove the reaction product, and moreover, the pump should be cleaned and the oil should be exchanged every time maintenance work is performed, thus bringing the manufacturing process to a halt. The activity rate is hence decreased. So long as the sealing oil is used in the vacuum pump, the oil is scattered from the downstream side to the upstream side, polluting the vacuum chamber and deteriorating the manufacturing efficiency.
In view of the above-described inconveniences, a positive displacement type screw vacuum pump has been developed and put into practical use as a dry pump which does not require the sealing oil. FIG. 14 is a side sectional view of an example of such screw vacuum pump. Within a housing 111 are provided two rotors 112, the rotary shafts of which are made parallel to each other. Spiral grooves are formed on the peripheral surfaces of the rotors 112. A space is defined when a recess portion (groove) 113a of one rotor and a projection 113b of the other rotor are meshed with each other. Thus, as the rotors 112 rotate, the capacity of the space changes, to cause sucking and discharging of the fluid.
In addition to the positive displacement type vacuum pump, a turbo type vacuum pump as shown in FIG. 15 has been developed.
The turbo type vacuum pump comprises a rotary shaft 150, a motor 15, ball bearings 152a and 152b, and a housing 153. A plurality of rotary disks 154 arranged in multiple stages is provided on the rotary shaft 150 and a spiral groove is formed on each of the surfaces of the rotary disks 154. An opposed surface 155 is formed on the fixed side of the pump, with a small gap provided therebetween to cause suction and discharge of the gas due to molecular drag operation of the spiral groove caused by the high speed rotation of the rotary shaft 150.
The positive displacement type vacuum pump and the turbo type vacuum pump have the following disadvantages:
In the conventional positive displacement type screw vacuum pump referred to above and shown in FIG. 14, the synchronous rotation of the rotors 112 is achieved by timing gears. That is, the rotation of a motor 115 is transmitted from a driving gear 116a to an intermediate gear 116b and further to one of the meshed timing gears 116c of the rotors 112. The phase of the rotating angles of both rotors 112 is adjusted by the engagement between the two timing gears 116c. Therefore, since the screw vacuum pump uses the gears both for transmission of the motor power and for synchronous rotation of the rotors as described hereinabove, a lubricating oil filled in a machine chamber 117 which houses the gears must be supplied to the gears. Moreover, a mechanical seal 119 should be provided between the machine chamber 117 and a fluid chamber 118 so as to prevent the lubricating oil from entering the chamber 118 where the rotors are accommodated.
The vacuum pump with two rotors in the above-described construction has disadvantages yet to be solved, in that (1) many gears are required for the power transmission and the synchronous rotation, i.e., many parts are required, resulting in a complicated structure of the apparatus, (2) a high speed operation cannot be expected and the apparatus is bulky in size since the rotors are synchronously rotated due to the contact maintained between the gears, (3) a mechanical seal must be regularly exchanged due to the abrasion thereof, such that a completely maintenance-free pump is not realized, (4) a large sliding torque due to the mechanical seal induces large mechanical losses, and so on.
Unlike the screw vacuum pump having two rotors, the turbo type vacuum pump has one rotor, namely, one rotary shaft. Accordingly, the rotary shaft can be driven at a high speed because the turbo type vacuum pump has no sliding mechanism allowing the two shafts to synchronously rotate. A clean dry pump can constituted by supplying lubricating oil to only the bearing section and providing a sealing section for preventing the penetration of the oil into the pump section.
Since the drag operation of the spiral groove allows the discharge performance of the pump to range from a viscous flow region to a molecular flow region, a vacuum can be generated to a degree of 10.sup.-5 torr.
As apparent from the graph of FIG. 4 showing, by a conventional example (1), characteristic data of the relationship between discharge speed and inlet pressure, in this kind of pump, i.e., the pump in FIG. 14, utilizing the molecular drag operation, the discharge speed is reduced to a great extent when the inlet pressure is in the range between atmospheric pressure and an intermediate degree of vacuum (10-3 to 10.sup.0 torr).
The generation of heat which occurs in the pump section in the above-described range of the inlet pressure makes it difficult to achieve continuous operation of the pump. As a result, the discharge period of time is long, which deteriorates the operational efficiency of the pump used in a semiconductor plant.